This family of snails is extremely unusual in that their shells consist of two separate hinged pieces or valves, which are joined by a ligament, and which look nothing like a normal snail shell, but instead look almost exactly like the two hinged valves of a clam, a bivalve mollusk, a very different class of animals.

In the past the Juliidae were known only from fossil shells, and not surprisingly these fossils were interpreted as being the shells of bivalves. Julia, which is the type genus of the family, was named in 1862 by Augustus Addison Gould, who described it as a bivalve genus. Julliidae are known from the Eocene period to the Recent, but they probably first appeared during the Paleocene.[2]

Taxonomy as "bivalves"

These bivalved gastropods were for a long time only known from fossils and dead material. Because of this, they had been described as being somewhat atypical bivalves. In the late 19th century they were classified among the bivalves, within the family Mytilidae, the true mussels.[3]

The similarity of the shells of Juliidae to those of bivalves does not mean that these snails are closely related to bivalves; this is an example of convergent evolution.

Discovery of live animals

Up until the mid 20th century, these creatures were still considered to be bivalves. Then finally in 1959, living individuals of one species were collected on the green alga, Caulerpa, in Japan. It was immediately clearly visible that these animals are in fact unusual gastropods with a two-part shell. The first discovered live species of bivalved gastropod was Tamanovalva limax, described by Kawaguti & Baba (1959).[4]

Once the habitat, appearance, and life habits of these very small and inconspicuous animals were understood, in subsequent years researchers were able to find a number of other species and other genera in different parts of the world, also living on various species of Caulerpa.

Description

Drawing of the interior of the left valve of the shell of "Julia borbonica".

Drawing of the interior of the right valve of the shell of "Julia borbonica".

As Tryon (1884)[3] wrote in his description of the genus Julia: the shell is oblong, thick, and cordiform. The valves are closed, the margins entire and the valves are inequilateral. The lunule is deep circular, projecting into the interior of the right valve, the left valve is in the same place furnished with dentiform tubercles. The hinge line is simple and arched. The ligament is external and narrow. There are two muscle scars which are unequal and subcentral.

These animals have two valves, and the soft parts can be completely withdrawn inside the shell.[7] The two valves are usually thin and translucent.

The body of the live animals is in most cases green (as it is in many sacoglossans), and in many species the individual appears green in totality. This, combined with the very small overall size, makes the animal hard to see on the green algae on which it lives. This ability serves as crypsis, especially as cryptic coloration (camouflage). In two species [1] and [2] the camouflage is even more complete: the mantle of the animal is patterned in a way that very closely resembles the structure of the alga on which it lives.

The empty valves of the shells of these animals are in some cases green, in other cases brownish-green or yellow, and in yet others, colorless. The species Julia zebra has shells that are finely striped with brown and blotched with white. [3]

While Jensen (2007)[12] recognized two recent genera with a few subgenera in the genus Berthelinia. Recent species listed here are based on Jensen (2007),[12] fossil genera and species are based on Le Renard (1996):[2]